The outage affected customers across the banking group's brands, including NatWest and Ulster Bank, with users reporting problems accessing ATMs as well as online services.

"We are disappointed that our customers faced disruption to banking services for a period yesterday evening, and apologise for that. All services are now running as normal again," RBS said in a statement.

"Between roughly 9-11pm customers were not able to access online banking and also had problems with ATM use and point of service payments."

The latest outage follows a major payments glitch last June when the NatWest and RBS systems failed to update customer balances overnight, impacting millions of customer payments for several days. The following month some NatWest customers could not access online banking or use debit cards.

Wednesday's glitch, however, was unrelated to those problems, according to RBS.

"This problem was caused by a hardware fault and was not related to the issues we experienced last summer. It was much easier to fix, though clearly an unacceptable failure," the bank said in a statement to ZDNet on Thursday.

"Any customer who was left out of pocket due to this outage should get in touch so we can put things right for them."

After the outages last June, the UK's banking regulator, the Financial Services Authority, was considering pressuring British banks to update their aging IT systems and RBS was thought to be facing the industry's first fine for an IT failure, according to a Financial Times report at the time.

A spokesperson for the FSA declined to comment on whether it was considering action over the latest outage.

"We are aware of the problem and we are talking with the bank," an FSA spokesperson told ZDNet.